Earth celebrates ten years of GRACE

Over a decade in orbit, the GRACE mission has mapped local variations in …

The two satellites of the GRACE mission keep careful track of the separation between them, which changes with local variations in gravity

Photograph by NASA

NASA's Gravity Recovery and Climate Experiment (GRACE) satellites just celebrated their ten year anniversary. Making precise measurements of the Earth's gravity field while traveling as a pair in orbit, the GRACE satellites have revolutionized several scientific fields and provided us with precise information about what's happening to the world's water and ice.

Launched March 17, 2002, the twin GRACEs orbit 500 km above the Earth with a separation of approximately 200 km. They work by using a microwave ranging system to measure differences in their separation as small as a hundredth of the thickness of a human hair. Using a high-quality GPS, they relay variations in separation along with their location down to Earthbound tracking stations.

The results have been an unprecedented view of the local gravity conditions. Water has plenty of mass, and GRACE can detect differences in groundwater with astounding accuracy, along with improvements in the precision of the geoid (a model of the Earth's gravity field) of between 10- to 100-fold. Measurements of ocean bottom pressure surprised oceanographers, and GRACE even profiles the global water vapor content of the Earth's atmosphere.

Although the satellites were originally designed to provide a resolution on the Earth's surface of about 400 km, researchers were able to improve this to 322 km. The GRACE data has been so valuable that the original five-year mission has now been extended for as long as the satellites hold out. The position of the two satellite was swapped in December 2005 to even out the wear from atmospheric drag, and data is no longer collected during periods of eclipse to avoid depleting the satellites' worn batteries. Each sweep of the entire Earth requires roughly a month.

It was GRACE that determined that ice loss from the high Asian mountain ranges was only 4 billion tons a year, compared to the 50 billion tons of ground-based estimates. GRACE pegs global ice loss over the period from 2003 to 2010 at about 4.3 trillion tons, adding about 0.5 inches to the global sea level in eight years. According to a JPL news release, that's enough to cover the entire United States in a foot and a half of water. About a quarter of the ice loss came from areas outside of Greenland and Antarctica. An eight-year study of Antarctica using Grace data and published in Nature revealed that the Antarctic ice loss was accelerating.

Mapping water at large scales enables hydrologists to connect small water processes to larger ones. One example is the separation of the Amazon watershed from the watersheds lying to the north.

On the other side of the world, in Northern India, water researchers were unable to divine to where gigantic volumes of water were disappearing every year beneath Jaipur and New Delhi. Groundwater levels were dropping by about a foot per year. The volume of missing water over a seven-year period added up to more than triple the volume of Lake Mead. GRACE data comes in quickly enough to show movement or other changes using monthly time series. Six months' worth of data from GRACE observations over India's northernmost states determined the water was going into human settlements and cropland.

The GRACE satellites have changed the way we look at water, shown us changes in the Earth's atmosphere, and given us hard data on melting rates for much of the world's ice. The satellites have been so successful that we've now sent a similar mission to the Moon. Although there is no GPS system available for the Moon, GRAIL, the Gravity Recovery and Interior Laboratory mission, has just begun to return data.